1. Field of the Invention
The present invention relates to a method for correcting the path of a gas laser beam which can be used in argon ion gas lasers, etc., used in a laser exposure process for inscribing ultra-high-precision images in gravure printing plates, etc., or carving pits in CDs etc., using a gas laser.
2. Prior Art
Conventionally, devices which are used to inscribe ultra-high-precision images in gravure printing plates, etc., by means of an argon ion gas laser are constructed as follows:
A single laser beam is conducted over a distance of 1 meter or more along a path turning in various directions by reflection from a multiple number of lenses. This laser beam is directed at a prescribed angle of incidence into a beam splitter which consists of a multi-coated half-mirror with transmissivities that vary according to a numerical series, or a beam splitter which is formed by stacking a multiple number of half-mirrors whose transmissivities vary according to a numerical series, so that the laser beam is split into approximately 20 beams of equal intensity, which are arranged in a row in the form of a connected chain.
The system is designed so that each beam can be passed through a multi-type light modulator which is driven independently in accordance with the image data and an autofocus lens, and directed onto the image-receiving medium photosensitive drum or silver salt film wrapped on a drum. Laser exposure is performed while scanning this medium.
However, in the case of a beam output from a gas laser, the mirrors used for pumping at both ends of the plasma tube are slightly shifted when the temperature of the room in which the gas laser is installed changes, or when vibrations originating in the rotating image-receiving medium or optical system act on the mirrors. As a result, the path of the laser beam output from the gas laser shifts as the laser is used.
Although this discrepancy may be small at the mirrors located near the gas laser, the discrepancy increases with the distance from the laser and becomes large in the mirrors which are distant from the laser. Accordingly, the beam is shifted considerably from the center of any lens that it passes through, resulting in serious distortion of the beam.
Especially in the case of a beam splitter, an ultra-high degree of precision is required in the angle of incidence of the beam. If there is a discrepancy in the beam path, a chain-form overlapping of the beams cannot be obtained, and the quantity of light also becomes insufficient. As a result, satisfactory ultra-precise image formation cannot be obtained in such prior art laser exposure devices used for gravure images, etc.